In the liver cell, the fastest system involved with regulation of cytoplasmic Ca2+ is the mitochondrial Ca2+ transport system. Several mechanisms have been found to be involved with mitochondrial Ca2+ transport. One of them, the Na+-independent Ca2+ efflux mechanism has neither been convincingly identified nor well characterized in published work. This mechanism is felt to play a crucial, central role in regulation of cytosolic Ca2+. Several possible hypotheses as to the nature of the Na+ independent Ca2+ efflux mechanism have been eliminated by earlier work. We show within this application preliminary data, which strongly supports the proposition that the Na+-independent Ca2+ efflux mechanism is active. This new data also supports the view that the properties of the Na+-independent Ca2+ efflux mechanism have geen disguised in much past work by induction of a membrane permeability transformation to a state of higher permeability by the conditions of some efflux experiments. We propose to systematically test those agents reported to protect against the permeability transformation in an effort to find conditions in which Ca2+ efflux under de-energized conditions can be studied without induction of the transformation. We further propose: (1) to study the inhibition of Ca2+ efflux by metabolic inhibitors. (2) to study the response of the efflux mechanism under both energized and de-energized conditions to changes in the Ca2+ chemical potential. (3) to study competition for efflux between Mn2+, Sr2+, and Ca2+. (4) to study Ca2+:Ca2+, Mn2+:Ca2+, and Sr2+:Ca2+ exchange under both energized and de-energized conditions. (5) to study the energization properties of mitochondria as they pass through the permeability transformation with a new cell sorter technique. In addition, we propose to determine if the Na+-dependent Ca2+ efflux mechanism sometimes seen in liver mitochondria is like that of heart mitochondria.